1. Field of the Invention
This invention relates to a polyvinylidene fluoride film having high dielectric strength and to a process for producing said film.
2. Discussion of the Prior Art
Polyvinylidene fluoride films are unique dielectric materials having piezoelectric and pyroelectric properties as well as a high dielectric constant. It has been desired to increase their dielectric strength in order to improve their electric properties. However, because the dielectric breakdown phenomenon of polyvinylidene fluoride films is complex as compared with other polymeric films, it is extremely difficult to increase the dielectric strength of such film. The results of early investigations have shown that when foreign materials of visible size, popularly called "fish eyes", are present in a polyvinylidene fluoride film, dielectric breakdown occurs at the sites of these visible foreign materials. However, breakdown is known to occur elsewhere and there are quite a few theories for the dielectric breakdown which occurs at the other sites in the film where no such visible foreign material exists, but no definite cause has yet been determined. One possible explanation is the presence of invisible minute foreign materials which can be located by an optical microscope. Voids or scratches generated during molding, and ionic impurities generated by decomposition during polymerization or molding have also been considered to be causes. It is thought that since the dielectric strength of a conventional polyvinylidene fluoride film is 8 MV/cm at room temperature and 6 MV/cm at 100.degree. C. which is higher than that of other polymers, the dielectric strength of polyvinylidene fluoride films has reached an upper limit and further improvement is not possible.
Due to the complexity of the problem, no method for improving the dielectric strength of polyvinylidene fluoride films has been reported.